I am using custom form 7
I want that, If anyone clicks the mail button under single post in wordpress, it will automatically redirect him to contact-us page and then subject of the mail will be the post title.
How can I do that?
Haven't used it before, but this plugin claims to work for your purposes: https://wordpress.org/plugins/contact-form-7-dynamic-text-extension/
You will then create your mail link to contain the post title via a query parameter with (maybe with http_build_str()), so that your URL becomes like:
http://example.com/contact-us/?title=my+post+title
After which you have to create the new CF7 tag as such, to auto-populate with the $_GET variable.
[dynamictext post_subject "CF7_GET key='title'"]